3GPP Long Term Evolution (LTE) is rapidly emerging as the world's most dominant 4G mobile broadband technology. To further extend the performance and capabilities of LTE radio access technology, 3GPP has been continuously working on the further evolutions of LTE to meet the demands for even higher data rates and network capacity.
LTE can operate in FDD (Frequency Division Duplex) or TDD (Time Division Duplex) mode, and TDD mode offers flexible deployments without requiring a pair of spectrum resources. Currently, LTE TDD allows for asymmetric uplink-downlink allocations by providing seven different semi-statically configured uplink-downlink (UL-DL) configurations. The semi-static allocation may or may not match the instantaneous traffic situation. The mechanisms, in which sub-frames may be allocated for either an uplink (UL) or a downlink (DL) transmission flexibly according to actual requirements, such as the instantaneous traffic situation, which is referred to as dynamic TDD for simplicity hereinafter, has been studied in 3GPP (3GPP TR 36.828 v11.0.0, Evolved Universal Terrestrial Radio Access (E-UTRA); Further enhancements to LTE Time Division Duplex (TDD) for Downlink-Uplink (DL-UL) interference management and traffic adaptation (Release 11), 2012 June). Ministry of Industry and Information Technology (MIIT) of China also shows great interest and identifies it as one of the key features to improve the performance in hot spot and indoor scenario.
In presence of dynamic TDD, when a cell as neighboring one schedules an UL transmission in a flexible sub-frame for a UE (here, the “flexible sub-frame” indicates a sub-frame which may be used for link transmission in a neighboring cell whose direction is different from that in a serving cell, and thus a conflict with the corresponding sub-frame used for the serving cell, i.e., a UL-to-DL interference, may exist, since the flexible sub-frame may be allocated for either a UL or a DL transmission flexibly based on actual requirements), the DL performance of UE(s) in the serving cell close to that UE in the neighboring cell would be seriously degraded, if the UE(s) are scheduled in the same sub-frame due to UE-to-UE interference, which is of great difference from that in a normal DL sub-frame with a fixed frame configuration. Therefore, there exists a problem that there are different UL to DL interference levels in presence of dynamic TDD, which seriously degrades the DL performance.
At the UE side, both a physical channel and the interference may be estimated from cell-specific reference signals which may reflect the interference from neighboring cells using the proprietary algorithms such as interpolation in frequency and time domains based on the classic minimum mean square error (MMSE) criteria, and filtered crossing all DL sub-frames for a legacy UE to assist the demodulation of all common channels, such as PDCCH and PHICH, and PDSCH with transmission modes from 1 to 6. The processing procedure is presented in FIG. 1, where the physical channel and interference are both estimated from cell-specific reference signals for the current, i.e., instantaneous, DL sub-frame. To obtain the more accurate interference estimation without instability, filters in the time domain crossing consecutive DL sub-frames may be always introduced to smooth the estimated value, which may be denoted by the following equations:H(n)=(1−αH)·H(n−1)+αH·H(n),  (1)I(n)=(1−αi)·I(n−1)+αi·I(n),  (2)where H(n) and H(n) denote the estimated instantaneous physical channel and the filtered physical channel for the nth DL sub-frame, respectively; I(n) and Ī(n) denote the estimated instantaneous interference power and the filtered estimated interference power for the nth sub-frame, respectively; n denotes a sequential number of the DL sub-frame; and αH and αI are forgetting factors for the physical channel and the interference, respectively, which are typically selected as a small number, e.g., 0.05, to guarantee the filtered value as more accurate as possible. The values of αH and αI may be designed in consideration of a system performance and variability in the channel which depends on UE velocity.
For the physical channel, the filtering for the estimated physical channel across the sub-frames may better improve the accuracy of a channel analyzer to assist channel estimation, so that the filtering across the sub-frames is always used for the channel estimation. The noisy estimation is expected to be removed by filtering for almost the same physical channel characteristics.
However, the filtering for the estimated interference across the sub-frames is not mandatory, and is only necessary in some cases. For example, if the interference originates from the DL normal sub-frames of the neighboring cells, which includes 1) the sub-frames in all the cells configured to use the same UL-DL sub-frame configuration in a case of high system load and 2) the sub-frames which have the same direction, i.e. DL, in both the serving and neighboring cells, the interference statistic having the same property may be better estimated and filtered according to Equation (2) from the cell-specific reference signals in case of filtering across the sub-frames.
However, with dynamic TDD, the UL-DL sub-frame configurations in the neighboring cells may be different from that in the serving cell. That is, the sub-frames with the same numbers may be configured for the UL transmission and the DL transmission in the neighboring cells, respectively, which thus may be referred to as flexible sub-frames. When the neighboring cell schedules the UL transmission in the flexible sub-frames for its resident UE(s), the DL performance of UE(s) in the serving cell close to that UE in the neighboring cell would be seriously degraded due to the UE-to-UE interference, which is of great difference that in the normal DL sub-frames. For the UE(s) in the serving cell, the characteristics of the experienced UE-to-UE interference in the flexible sub-frames are quite different from the DL to DL (i.e., base station to UE) in the normal DL sub-frames. As a consequence, the interference on the flexible sub-frames may not be filtered from the cell-specific reference signals together with the normal sub-frames with the same filter functions denoted in Equations (1) and (2). The strong interference from the flexible sub-frame may interrupt the correctness of the filtered value, such as to spread the biased interference value to all DL sub-frames, which would degrade the DL performance seriously. That is, when the filters across the sub-frames are deployed in the victim UE(s), the DL performance of all sub-frames are seriously degraded due to that the filtering loops are polluted by the strong UE-to-UE interference.
FIG. 2 as an example shows UL-DL sub-frame configuration 2 and sub-frame configuration 1 for Cell 30-1 and Cell 30-2, respectively, where the configuration number is referred to the 3GPP specification (3GPP TS 36.211 v10.3.0, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 10), 2011 September). The DL sub-frames of Sub-frame #3 and #8 of UE 32-1 served by Cell 30-1 in the cell edge may be seriously interfered by the UL transmission in those Sub-frame #3 and #8 of the UE32-2 served by Cell 30-2, when UE 32-1 and UE 32-2 are close to the cell border between Cell 30-1 and Cell 30-2, as illustrated in FIG. 3. To identify the impact, a small scale field trial with two pico cells and one UE in either cell has been set up. The test results of the UE 32-1 DL throughput on each DL sub-frame in one frame are shown in FIG. 4, which shows that not only Sub-frames #3 and #8 in Cell 30-1 are degraded, but also all normal DL sub-frames are degraded with the existing interference filtering scheme, which is aligned with the theoretical analysis above. Therefore, it is necessary to improve the filtering module for the estimated channel and the estimated interference (if necessary) in the UE.